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Positrons for Applied Science & Materials Science K.G. Lynn and M.H. Weber and many others!! Washington State University, Pullman, WA JPOS 09 International.

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Presentation on theme: "Positrons for Applied Science & Materials Science K.G. Lynn and M.H. Weber and many others!! Washington State University, Pullman, WA JPOS 09 International."— Presentation transcript:

1 Positrons for Applied Science & Materials Science K.G. Lynn and M.H. Weber and many others!! Washington State University, Pullman, WA JPOS 09 International Workshop on Positrons at Jefferson Lab Thomas Jefferson National Accelerator Facility Newport News, VA March 25-27, 2009

2 My Concerns in a low energy positron facility Intense positron sources have not fulfilled its promise to DOE/NSF The intense sources have been small groups trying to move into a larger facilities based on the researchers interests and lacked the support of the facilitiy and funding agency. The beams that have operated have not provided the needed user support and users have gone elsewhere. Neither the brightness nor the intensity has been routinely been achieved If the Jefferson Lab is planning this facility a real commitment is needed from OS/DOE and local management

3 Number 1 : Positrons made the Newsweek hitlist

4 The positron’s death  = g (p  ) red shifted blue shifted Eventually, in our world of matter, the positron will annihilate with an electron. Two (or rarely three) photons (gamma rays) emerge. The number of electrons (density) determines how fast this occurs Basic laws of nature (physics) force certain conditions: 2 gammas in opposite direction with small changes in energy (Doppler shifts) and direction. Doppler shifts Angular deviation from opposite JPOS 09 Newport News (March 2009)

5 Positron characteristics Unique quantum numbers –No exchange at the present time Annihilation with electrons radiation can be detected –Little interaction with specimen after annihilation Electron momentum encoded in  -rays –Doppler broadening –Angular correlation Lifetime is electron density dependent –Positron lifetimes

6 Hits in “Defects” Vacancy formation enthalpies in metals (90%) (1975- present) Voids in neutron irradiation and deformation of metals Observation of vacancy migration at stage III (1980) –Major controversy resolved Vacancies observations in compound semiconductors (1990) Vacancy character of EL2 in GaAs (1993) Role of defects in hi-Tc superconductors ( ) Open volume measurements in polymers (ongoing) –Gas diffusion, Mechanical properties, Aging Defects at semiconductor interfaces (ongoing)

7 Annihilation at high relative momentum 2D spectrum: x: p-parallel Doppler shift y: Sum energy rest mass + kinetic energy 1022 keV 1092 keV 0 keV 340 keV => 91.3 a.u keV JPOS 09 Newport News (March 2009)

8 Channeling Angle Normalized Yield

9 Positron Holography (Never fulfilled) CdSe - Electron-electron interaction - Multi-layer contribution - One positron at a time - Topmost layer only Now: with electrons  Future: with positrons “ If positrons were routinely available, all diffraction would be done with them ” S.Y. Tong

10 Fermi Surfaces Ytterbium Experiment Theory Now: 16 dataset  Future: Super ACAR 1 shot and depth profiles Resolution limited by acquisition time

11 Quantum dots 1.8 nm 6.0 nm 4.5 nm “baseline” JPOS 09 Newport News (March 2009)

12 Fe Cu Zero point motion energy Potential well in Fe e+e+ Cu in Fe Precipitates-Critical in Reactor Steels

13 Atomic scale defects Missing atoms in crystals are called vacancies They play a key role in the properties of many metals, semiconductors and insulators How to tell the difference between impurities and dopants –One makes the PC work the other turns it to a pile of junk Understanding them drives progress –Electronics, solar cells, sensors, optics, detectors (airports), lasers –Silicon, silicon carbide, ZnO, GaN, GaAs, YAG,… –Lasers to cut steel, transparent conductors for monitors, sunlight to electricity, longer lasting cell phones, more gigabytes on DVDs beyond Blue-ray, shorter queues at airport baggage scanners… JPOS 09 Newport News (March 2009)

14 Trapping in negative or missing atoms   -direction (a.u.) direction (a.u.) direction (a.u.) Delocalized Bloch state Localized trapped state

15 A positron and many electrons Doppler broadening Conduction electrons: delocalized; low momentum Bound electrons: localized; high momentum Potential of atomic cores JPOS 09 Newport News (March 2009)

16 A positron “likes” vacancy Doppler broadening Conduction electrons: delocalized; low momentum Bound electrons: localized; high momentum JPOS 09 Newport News (March 2009)

17 Temperature (K) Open volume parameter TcTc Vacancy formation energy Mo Now: 1D depth profile  Future: 3D map with lifetime

18 Depth profiles Now: layer averaged  Future: 3D map with nm 3 resolution

19 SiO 2 -Si interface Ps trapped in microvoids at the interface With broad component Without broad component

20 Bulk material level Colloidal silica (50 nm) JPOS 09 Newport News (March 2009)

21 Defects in matter The mesh represents electrons “flowing” around atoms in silicon. The atoms are indicated by the red spheres. One atoms is missing and a different atom (green) is replacing a neighboring silicon. This is hard to “see” but can be detected with positrons. JPOS 09 Newport News (March 2009)

22 Looking for defects Doppler shift momentum Total energy Highly porous material JPOS 09 Newport News (March 2009)

23 Chemical environment Coincident positron annihilation sensitive to core electrons Now: 12 hours for 1 1 selected depth  Future: within hours a full depth profile x 1/2

24 Micro probes News item in Nature vol. 412, p.764 (2001) W. Triftshauser et al, Phys. Rev. Lett. 87, (2001) Combined positron (1-5) and electron (7-6) Microscope (9-10) to probe cracks in metals (11,13). An electrical prism (6) switched between electrons and positrons to combine electron microscope and defect images. Greif et al, Appl. Phys. Lett. vol 71, p (1997) Positron probe that Measures the electron density of patterns on silicon with 2 micrometer resolution JPOS 09 Newport News (March 2009)

25 Cracks Lifetime scale (ps) Dislocations Void Matrix

26 The future of Defects 2D lifetime maps Simulation of the future with e + Vacancies Dislocations Matrix Precipitate Small void TEM Lifetime scale (ps)

27 Stress-Are you feeling some?? stress relieved under stress Direct observation of dislocations in metals during elastic deformation Lifetime Intensity Now: stop frame  Future: movie

28 Lifetime apparatus Stop:  detector discriminator Data collecting computer positron beam Start: e - detector discriminator JPOS 09 Newport News (March 2009)

29 Positron lifetime No pores big space between molecules large pores JPOS 09 Newport News (March 2009)

30 Positron Lifetime Unit-cell volume (a.u.) Positron lifetime (ps) Now: bulk averaged  Future: 3D map

31 Positronium in Voids & Open Porosity porosity interconnectivity surface + JPOS 09 Newport News (March 2009)

32 o-Positronium Lifetime Pore radius o-Ps lifetime SILICAGEL ALUMINAGEL POROUS VYCOR GLASS SILICAGEL SODALITE MS-4A MS-5A  -CYCRO DEXISTRIN MS-3A 13X 3A 4A 13X 4A 5A Now: bulk averaged  Future: 3D map

33 Separating closed and open porosities (at 2 keV) Open/closed porosity differ qualitatively : Closed vs open porosity JPOS 09 Newport News (March 2009)

34 Percolation Threshold; Open Porosity JPOS 09 Newport News (March 2009)

35 Two pore diameters JPOS 09 Newport News (March 2009)

36 Pores in materials The size of pores determines –what size molecules pass –how long a pill can deliver drugs –the function of fuel cells –the mechanical properties of plastics –how fast a computer can calculate –the purity of filtered water Filters, membranes, drug-delivery, microelectronics How to measure the size? –These are nanometers. JPOS 09 Newport News (March 2009)

37 Ce:YAG Boule JPOS 09 Newport News (March 2009)

38 JPOS 09 Newport News (March 2009)

39 JPOS 09 Newport News (March 2009)

40 #2 Zn A BCDE FG HIJ As rec.: clear ZnTi(H)Ti(D)Ti(H dep) Zn Ti(H dep)O2O2 Ref [24]#1 #3 JPOS 09 Newport News (March 2009)

41 Oxidation of a layer on Si layer Si

42 Zero Temperature Limit of 3  /2  ratio Extrapolate to 0 K Initial Amount of Ps  with  in T c.f results of Goworek. Increase in R due to increase in pore lifetimes  Less initial Ps but less pick-off  “Purification”: Greater relative intensity of self-annihilation Ps does not die out JPOS 09 Newport News (March 2009)


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